(1) Field of the Invention
The present invention relates, among others, to a semiconductor thin film used as a semiconductor element, a method for manufacturing the same, and a thin film transistor employing the same.
(2) Description of the Prior Art
Amorphous silicon thin films are conventionally being employed as functional semiconductor thin films for use in thin film semiconductor devices such as thin film transistors or thin film solar batteries. Thin film transistors employing amorphous silicon films as active layers are being materialized as switching elements for driving pixels in liquid crystal display devices or the like, and solar batteries employing amorphous silicon films as photoelectric converting layers are being materialized in consumer-oriented fields such as watches/clocks or calculators.
Transistors that are employed in liquid crystal displays of active matrix type are, for instance, transistors for switching respective pixels or high-mobility transistors for peripheral circuits for sending control signals, which are based on image information to be displayed, to respective pixel transistors. Among these, it was conventionally the case that TFTs using amorphous silicon hydride (a-Si:H) as active layers in pixel transistors while these TFTs were manufactured through plasma chemical vapor deposition (PCVD).
While such a-Si:H TFTs are advantaged in that they may be manufactured at a temperature of approximately 300xc2x0 C. in which light-transmitting glass substrates of low costs are well applicable, drawbacks are presented in that the mobility of n-type TFTs will be small ranging around 1 cm2/Vs while no practical mobility can be achieved in case of p-type TFTs so that such TFTs are not applicable to peripheral circuits. Thus, peripheral circuits were arranged by mounting IC chips onto substrates.
On the other hand, TFTs employing polycrystalline silicon (poly-Si) as active layers are advantaged in that these TFTs exhibit high mobility for both, n-type and p-type ones and that they are applicable also to peripheral circuits. However, when using poly-Si, it is necessary to form films through reduced-pressure CVD methods in which processes need to be performed in high-temperature conditions of not less than 600xc2x0 C. and thus to present a drawback that glass substrates of low costs could not be used.
Active research and developments related to techniques of manufacturing poly-Si (low temperature poly-Si) at low temperature in which glass substrates of low costs applicable are being made and materialized. One exemplary method is a method for manufacturing a polycrystalline film in which excimer laser beams with wavelengths existing in ultraviolet regions that are extremely highly absorbed by a-Si:H films are being irradiated onto a-Si:H films in a pulse-like manner for rapidly performing heat-melting and cooling of the a-Si:H film to achieve recrystallization (see Japanese Patent No. 2725669 and others). While this method enables it to form TFTs of high mobility at low temperature of not more than 600xc2x0 C. in which glass substrates are applicable, drawbacks of this method reside in the fact that it is difficult to form poly-Si films of large areas owing to the fact of utilizing laser beams, and that the productivity thereof will be inferior. Further, since a-Si:H films generally contain hydrogen of not less than 10 atom %, bumping of hydrogen will be caused through rapid heating using excimer laser beams to result in peeling of films or roughing of surfaces when using the films as they are, and it was necessary to perform an additional heat-treatment process of preliminarily removing hydrogen contained in the film.
It has been proposed for a technique for solving the above subjects as will be explained below as a technique of manufacturing crystalline silicon films at low temperature.
Japanese Patent Unexamined Publication No. 8-250438 (1996) discloses a method for forming a silicon thin film through catalytic CVD methods in which a catalyst is heated to not less than a melting point of Si, in which a part of molecules of a raw material gas is made to contact the heated catalyst for resolution, and in which film forming is performed through CVD and crystal growth is performed on a substrate. In this method, a part of fly-coming species will reach quite a high temperature, behave as if the substrate surface would be of high temperature, and it is considered that the polycrystalline silicon is formed at a low substrate temperature. Actually, the raw material gas is a mixed gas of silicon (Si) compound gas and other substances and the catalyst is heated through supplied electric power. By setting the following conditions to be suitable for making the silicon thin film that is made from depository species to be a polycrystalline thin film, a polycrystalline silicon thin film is formed on a substrate of low temperature: a pressure condition in which the pressure of a reaction chamber for generating the silicon thin film is set to be at low pressure; a condition for a mixing ratio of raw material gas in which the ratio of gas containing other substances to gas of silicon compounds is set to be larger; and a condition for supplied electric power for the catalyst in which the electric power to be supplied to the catalyst is set to obtain a catalyst temperature that is not less than the melting point of silicon.
A method for forming a crystalline silicon film through chemical vapor deposition process in which resolution is performed through plasma resolution utilizing high-frequency inductive coupled plasma (ICP) and in which the resolved raw material gas is used is disclosed in Japanese Patent Unexamined Publication No. 10-265212 (1998) and No. 11-74204 (1999). In such a method, electric power of high frequency is invested through electrodes (antenna) for generating ICP (reference should be made to xe2x80x9cApplied Physicsxe2x80x9d, Hidero Sugai, Vol. 63, No. 6, 1994, pp. 559-567) for generating high density plasma of the raw material gas and for performing film forming through resolution and high excitation of the raw material gas. It is obvious from the above Japanese Patent Unexamined Publication No. 10-265212 that it is possible to obtain a polycrystalline silicon thin film with a conductivity that is higher by an order of magnitude by setting the high frequency electric power to not less than 800 W while the conductivity of the film is degraded in case the pressure exceeds 6.65 PA (50 mTorr) so that no minor-crystalline or polycrystalline silicon thin film could be obtained.
A method for film forming through plasma CVD in which ion beams are irradiated from the exterior for forming a crystalline silicon film is disclosed in Japanese Patent Unexamined Publication No. 11-145062 (1999). In such a method, surface excitation effects can be obtained by irradiating ion beams of 0.1 kV to 40 kV on to a film forming surface to thereby obtain a silicon thin film exhibiting favorable crystallinity.
However, while it seems to be possible to manufacture crystalline silicon films of minor-crystalline or polycrystalline type at temperatures in which glass substrates are applicable by using the above methods, all of the crystalline silicon films that may be obtained by the above techniques assume crystal structures in which crystals are grown on substrates in columnar styles so that large concaves and convexes are formed on these surfaces as illustrated in FIG. 1. This is due to the fact that growth of crystalline silicon films is dependent on a balance between film deposition of silicon type radical and etching using hydrogen atoms or halogen type radical. More particularly, since an etching speed within grain boundaries including a large amount of weak Si combinations is faster than an etching speed within the grains, the grain boundaries are selectively etched to expose the grains, and it is considered that concaves and convexes of 20 nm to 100 nm are generated thereby. Such concaves and convexes formed on the surface cause degradations in characteristics and reliability of the semiconductor device. For instance, in case of thin film transistors of top gate type, concaves and convexes formed on interfaces between channels and gate insulating films cause degradations in characteristics owing to scattering of carriers and further in degradations in reliability owing to broken insulations that are caused through partial thinning of gate insulating films.
Moreover, an amorphous layer is formed proximate to the interface with the substrate. This is a silicon layer that has deposited until a crystal core is formed which is necessary for the crystalline silicon to deposit on the substrate surface. Since the thickness of the amorphous layer generally accounts to several tens of nm in the proximity of the interface, they may be of disadvantage in case of forming thin film transistors in which thin and high crystalline semiconductor films are required.
Grain boundaries further exhibit relatively large defects so that they are apt to oxidation while problems of decreases in ON currents or increases in OFF currents are caused owing to oxidation of polycrystalline silicon grain boundaries in case of devices such as thin film transistors.
The respective film forming techniques further presented the following drawbacks.
In the catalyst CVD method (Japanese Patent Unexamined Publication No. 8-250438), film forming is performed by using only active species (radicals) generated through thermal resolution in a gaseous state. As stated, not only resolution but also increasing the temperature (moving energy) of a part of radicals to be sufficiently large is achieved through contact with the thermal catalyst for forming the crystalline silicon layer without increasing the substrate temperature. For achieving this effect, it is necessary to clear the problem of investing electric power to the thermal catalyst for heating purposes such that the thermal catalyst reaches an extremely high temperature of 1,700 to 1,800xc2x0 C. This problem becomes especially remarkable in case of forming the film on substrates with large areas such as liquid crystal displays. The temperature thus needs to be set such that structures of devices and low-cost substrates made of glass or the like need to stand radiation generated through such thermal catalysts of high temperature.
In the method for forming a crystalline silicon film through PCVD using ICP (Japanese Patent Unexamined Publications No. 10-265212 and No. 11-74204), a crystalline silicon film is formed by generating high-density plasma of raw material gas owing to high frequency inductive coupling for performing more active resolution and excitation of raw material gas than in conventional plane-parallel type high frequency PCVD methods. However, it was found through Raman spectroscopy that the obtained film was not satisfactorily in view of crystallinity or film qualities, being either of minor-crystalline type (Japanese Patent Unexamined Publication No. 10-265212) or exhibiting large light conductivity (Japanese Patent Unexamined Publication No. 11-74204).
All of these techniques are techniques that have been made in view of high resolution and high excitation of raw material gas, and it is considered that the problem resides in lack of means for promoting crystallization and decreasing hydrogen densities in films. This is the reason why the amorphous film 21 is formed in proximity of the interface as illustrated in FIG. 1 owing to poor crystallinity of the film in proximity of the interface with the substrate.
Since ion beams of high acceleration are used in the method for irradiating ion beams onto the film forming surface from the exterior (Japanese Patent Unexamined Publication No. 11-145062), the entire device becomes a large-scaled one and it is difficult to uniformly treat a large surface are. It is further considered that many defects are generated in formed films since high-speed ion continuously hits against films during film forming.
In applying these as semiconductor elements of thin film transistors or solar batteries, it will be an important factor to perform control of crystallinity in proximities of interfaces with substrates and defect densities within films. None of the above-described conventional techniques are ready to cope with such subjects.
It is thus an object of the present invention to provide a semiconductor thin film and a manufacturing method thereof that can be directly formed on a substrate, that exhibits superior crystallinity at low temperature and low defect density, and that has a smooth surface.
It is another object thereof to provide a thin film transistor and a manufacturing method thereof that exhibits favorable characteristics as well as reliability and that is of favorable productivity.
It is still another object thereof to provide a liquid crystal display device and a manufacturing method thereof of improved image quality by forming a thin film transistor array exhibiting favorable characteristics and reliability on a glass substrate of large surface area.
It is still another object of the present invention to provide a thin film transistor of superior characteristics, a liquid crystal display device of superior image quality and a solar cell of high conversion efficiency by decreasing intermixing of oxygen impurities into a crystalline semiconductor film.
It is still another object of the present invention to provide a device for manufacturing a semiconductor thin film in which control of film forming speeds or crystallinity can be easily and reliably performed.
It is still another object of the present invention to provide a semiconductor thin film, a semiconductor device employing the same and manufacturing methods thereof in which it is possible to use substrates of low costs such as those made of glass.
It is still another object of the present invention to provide a method for manufacturing a semiconductor device such as a transmitting type liquid crystal display or the like in which a light-transmitting semiconductor device may be easily manufactured.
It is still another object of the present invention to provide a semiconductor thin film that exhibits high field-effect mobility of TFTs when compared with a-Si:H films.
It is still another object of the present invention to provide a semiconductor thin film that may be employed as an active layer of a TFT as it is.
It is still another object of the present invention to provide a device for manufacturing a semiconductor thin film with which the above semiconductor thin films and a semiconductor device employing the same may be easily manufactured.
For achieving one of the above objects, the invention as recited in embodiment 1 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas by employing a first energy, the method including at least an interfacial film forming process in which an interfacial film is formed on a surface of the substrate and a main body film growing process in which a semiconductor film is deposited and grown, wherein a second energy is added during the interfacial film forming process.
By the addition of the second energy during the interfacial film forming process as in the above arrangement, crystallinity in proximities of the substrate interface may be improved due to the addition of external energy through ion only on to the interfacial film.
By performing resolution of raw material gas including compositional elements of the semiconductor thin film by utilizing plasma, it is possible to further decrease the temperature of the substrate. It is preferable to generate the plasma through non-electrode discharge such as microwave plasma, helicon wave plasma, surface wave plasma, or electron cyclotron resonance plasma or through plane-parallel type capacitive coupling plasma with a power source frequency of 20 MHz to 100 MHz since the potential of plasma may be decreased thereby. It is more preferable to generate plasma through microwave plasma, surface wave plasma or plane-parallel type capacitive coupling plasma with a power source frequency of 20 MHz to 50 MHz, since the plasma potential will be not more than 30 V as it will be described later, and the growth of films on substrates of large surface areas may be easier performed.
According to the invention as recited in embodiment 2, the second energy of the invention as recited in embodiment 1 is ion energy.
According to the invention as recited in embodiment 3, the second energy of the invention as recited in embodiment 1 is optical energy.
For achieving one of the above objects, the invention as recited in embodiment 4 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas, the method including at least an interfacial film forming process in which an interfacial film is formed on the substrate and a main body film growing process in which a semiconductor film is deposited and grown, wherein ion collision energy applied on the interfacial film surface during the interfacial film forming process is restricted to be larger than ion collision energy applied on the semiconductor film surface during the main body film growing process.
For achieving one of the above objects, the invention as recited in embodiment 5 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas, the method including at least an interfacial film forming process in which an interfacial film is formed on the substrate and a main body film growing process in which a semiconductor film is deposited and grown, wherein ion flux applied on the interfacial film surface during the interfacial film forming process is restricted to be larger than ion flux applied on the semiconductor film surface during the main body film growing process.
By restricting the ion collision energy applied on the interfacial film surface during the film forming process to be larger than the ion collision energy applied on the semiconductor film surface during the main body film growing process or by restricting the ion flux applied on the interfacial film surface during the interfacial film forming process to be larger than the ion flux applied on the semiconductor film surface during the main body film growing process as in the above arrangements, external energy through ion may be added only onto the interfacial film for improving the crystallinity in proximities of the interface of the substrate (interfacial film). Further, by restricting irradiation of ion only onto the interfacial film, it is possible to reduce defects formed in the semiconductor film (main body film) since the main body film includes relatively more hydrogen than the interfacial film, and degradations in electric characteristics as a ready device can be restricted. With this arrangement, it is made possible to directly form a crystalline semiconductor film in a uniform manner with few defects also at a temperature enabling usage of a substrate of low costs such as one made of glass.
Note that the term xe2x80x9cion fluxxe2x80x9d denotes a radiation value of ion per unit time and unit area.
According to the invention as recited in embodiment 6, the ion collision energy applied to the interfacial film surface during the interfacial film forming process of the invention as recited in embodiment 4 is not less than 30 eV and not more than 1 keV while the ion collision energy applied to the semiconductor film surface during the main body film growing process is not more than 30 eV.
By restricting the ion collision energy during the interfacial film forming process into the above range of not less than 30 eV and not more than 1 keV, it is possible to perform energy control by bias impression onto the substrate whereby it is enabled it to ignore the effect of spattering through ion. On the other hand, the reason for restricting the ion collision energy during the main body film growing process to not more than 30 eV is that the defect density of the semiconductor film will be increased in case this value exceeds 30 eV.
For achieving one of the above objects, the invention as recited in embodiment 7 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas, the method including at least a main body film growing process in which a semiconductor film is deposited and grown on the substrate and a smoothing process for smoothing the surface of the semiconductor film, wherein ion collision energy applied on the semiconductor film surface during the smoothing process is restricted to be larger than ion collision energy applied on the semiconductor film surface during the main body film growing process.
For achieving one of the above objects, the invention as recited in embodiment 8 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas by employing a first energy, the method including at least a main body film growing process in which a semiconductor film is deposited and grown on the substrate and a smoothing process for smoothing the surface of the semiconductor film, wherein ion flux applied on the semiconductor film surface during the smoothing process is restricted to be larger than ion flux applied on the semiconductor film surface during the main body film growing process.
By restricting the ion collision energy applied on the semiconductor film surface during the smoothing process to be larger than the ion collision energy applied on the semiconductor film surface during the main body film growing process or by restricting the ion flux applied on the semiconductor film surface during the smoothing process to be larger than the ion flux applied on the semiconductor film surface during the main body film growing process, it is possible to easily perform smoothing of the surface of the semiconductor film and to decrease defects of the main body film.
According to the invention as recited in embodiment 9, the ion collision energy applied to the semiconductor film surface during the smoothing process of the invention as recited in embodiment 8 is not less than 36 eV and not more than 1 keV while the ion collision energy applied to the semiconductor film surface during the main body film growing process is not more than 30 eV.
By restricting the ion collision energy during the smoothing process into the above range of not less than 36 eV and not more than 1 keV, it is possible to perform energy control by bias impression onto the substrate whereby it is enabled it to ignore the effect of spattering through ion. On the other hand, the reason for restricting the ion collision energy during the main body film growing process to not more than 30 eV is that the defect density of the semiconductor film will be increased in case this value exceeds 30 eV.
For achieving one of the above objects, the invention as recited in embodiment 10 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material, the method including at least an interfacial film forming process in which an interfacial film is formed on the substrate, a main body film growing process in which a semiconductor film is deposited and grown, and a smoothing process for smoothing the surface of the semiconductor film, wherein ion collision energy applied on the interfacial film surface or the semiconductor film surface during the interfacial film forming process and the smoothing process is restricted to be larger than ion collision energy applied on the semiconductor film surface during the main body film growing process.
By restricting the ion collision energy applied onto the semiconductor film surface during the interfacial film forming process and the smoothing process to be larger than the ion collision energy applied on the semiconductor film surface during the main body film growing process, it is possible to improve crystallinity of the interface film, to easily perform smoothing of the surface of the semiconductor film and to decrease defects of the main body film.
According to the invention as recited in embodiment 11, the ion collision energy applied to the interfacial film surface during the interfacial film forming process of the invention as recited in embodiment 10 is not less than 30 eV and not more than 1 keV while the ion collision energy applied to the semiconductor film surface during the main body film growing process is not more than 30 eV and the ion collision energy applied on the semiconductor film surface during the smoothing process is not less than 36 eV and not more than 1 keV.
By restricting the ion collision energy during the smoothing process into the above range of not less than 36 eV and not more than 1 keV and the ion collision energy applied onto the surface of the substrate during the interfacial film forming process to be not less than 30 eV and not more than 1 keV, it is possible to perform energy control by bias impression onto the substrate whereby it is enabled it to ignore the effect of spattering through ion. On the other hand, the reason for restricting the ion collision energy during the main body film growing process to not more than 30 eV is that the defect density of the semiconductor film will be increased in case this value exceeds 30 eV.
For achieving one of the above objects, the invention as recited in embodiment 12 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas, the method including at least an interfacial film forming process in which an interfacial film is formed on the substrate, a main body film growing process in which a semiconductor film is deposited and grown, and a smoothing process for smoothing the surface of the semiconductor film, wherein ion flux applied on the interfacial film surface or the semiconductor film surface during the interfacial film forming process and the smoothing process is restricted to be larger than ion flux applied on the semiconductor film surface during the main body film growing process.
By restricting the ion flux applied on the semiconductor film surface during the interfacial film forming process and the smoothing process to be larger than the ion flux applied on the semiconductor film surface during the main body film growing process, it is possible to improve crystallinity of the interfacial film, to easily perform smoothing of the surface of the semiconductor film and to decrease defects of the main body film.
According to the invention as recited in embodiment 13, the ion collision energy as recited in claim 4 is controlled through a potential of the substrate.
According to the invention as recited in embodiment 14, the ion collision energy as recited in claim 4 is controlled through a potential of the plasma.
For achieving one of the above objects, the invention as recited in embodiment 15 of the present invention relates to a device for manufacturing a semiconductor thin film of plane-parallel type comprising a plasma source for performing plasma resolution of raw material gas, a bias electrode for controlling ion energy upon plasma resolution for irradiating the same onto a substrate, and a substrate heating means, wherein a power source frequency of the plasma source is not less than 20 MHz, and an ion energy that is controlled by the bias electrode not less than 30 eV and not more than 1 keV.
Here, the term xe2x80x9cion energyxe2x80x9d denotes a difference between a potential of plasma including the ion and a potential of the substrate, and ion is accelerated owing to the presence of such difference.
For achieving one of the above objects, the invention as recited in embodiment 16 of the present invention is comprised of a plasma source for performing plasma resolution of raw material gas, a support base for supporting a substrate, and an electrode for controlling a potential of plasma.
By the provision of the electrode for controlling the potential of the plasma as in the above arrangement, ion energy may be easily and reliably controlled.
According to the invention as recited in embodiment 17, at least a part of the plasma and the electrode of the invention as recited in embodiment 16 are in contact with each other.
According to the invention as recited in embodiment 18, the electrode of the invention as recited in embodiment 16 is a frame-like electrode that comes in contact with peripheral edge portions of the plasma.
Since the plasma is a conductive body, it will be suffice if at least a part of the plasma is partially in contact with the electrode, while it is possible to reliably perform control of ion energy when the electrode is formed as a frame-like electrode that comes in contact with the peripheral edge portions of the plasma.
For achieving one of the above objects, the invention as recited in embodiment 19 of the present invention relates to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas, the method including at least an interfacial film forming process in which an interfacial film is formed on the substrate and a main body film growing process in which a semiconductor film is deposited and grown, wherein a temperature rising speed of the interfacial film that is heated by means of an optical heating means during the interfacial film forming process is not less than 6xc2x0 C./sec.
By setting the temperature rising speed of the interfacial film to be not less than 6xc2x0 C./sec, film forming may be completed in a short time.
According to the invention as recited in embodiment 20, a heating temperature during the interfacial film forming process of the invention as recited in embodiment 19 is not less than a temperature of the substrate during the film forming process and not more than 1,200xc2x0 C.
By making such restrictions, it is possible to cope with problems in which crystallization of the interfacial film becomes insufficient in case the heating temperature is less than the temperature of the semiconductor film during the semiconductor film forming process and in which a substrate of low melting point such as one made of glass may be deformed in case the temperature exceeds 1,200xc2x0 C.
According to the invention as recited in embodiment 21, light that is irradiated during the interfacial film forming process of the invention as recited in embodiment 19 includes light having a wavelength of at least not more than 300 nm.
In case light having a wavelength of at least not more than 300 nm is included in irradiated light as in the above arrangement, only the semiconductor film on the substrate may be selectively heated.
For achieving one of the above objects, the invention as recited in embodiment 22 of the present invention is related to a device for manufacturing a semiconductor thin film comprising a plasma source for performing plasma resolution of raw material gas, a substrate, and an optical heating means for heating by irradiating light onto an interfacial film formed on an interface of the substrate, wherein a potential of plasma generated by the plasma source is not more than 30 V, and the interfacial formed on the substrate is heated at a temperature rising speed of not less than 6xc2x0 C./sec.
According to the invention as recited in embodiment 23, the heating means of the invention as recited in embodiment 22 is comprised of the optical heating means only.
According to the invention as recited in embodiment 24, the device of the invention as recited in embodiment 22 further comprises a heating means for the substrate that is incorporated in a support base for the substrate in addition to the optical heating means.
By the above arrangement, it is possible to increase the temperature of the substrate in a more rapid manner.
According to the invention as recited in embodiment 25, light that is emitted from the optical heating means of the invention as recited in embodiment 22 includes light having a wavelength of at least not more than 300 nm.
For achieving one of the above objects, the invention as recited in embodiment 26 of the present invention is related to a method for manufacturing a semiconductor thin film on a substrate through plasma resolution of raw material gas, the method including at least an impurity adding process of adding an impurity that affects surface reaction of the surface of the substrate at the time of performing interfacial film forming and a main body film forming process in which a semiconductor film is deposited and grown, wherein the impurity adding process and the main body film forming process are performed without exposure to atmosphere.
By the inclusion of an impurity that affects surface reaction of the surface of the substrate at the time of performing interfacial film forming, it is possible to directly form crystalline semiconductor film that is uniform and includes few defects also at a temperature in which a substrate of low costs such as one made of glass may be used. When performing the impurity adding process and the main body film forming process without exposure to atmosphere, it is possible to restrict oxidation of the interfacial film.
According to the invention as recited in embodiment 27, the impurity of the invention as recited in embodiment 26 exhibits catalytic actions.
According to the invention as recited in embodiment 28, the impurity of the invention as recited in embodiment 26 is at least one selected from a group consisting of phosphorus, boron, nickel, and palladium, and an amount of addition of the impurity is in a range of 1xc3x971016 cmxe2x88x923 to 1xc3x971019 cmxe2x88x923.
The reason for setting the amount of addition of the impurity to be in the range of 1xc3x971016 cmxe2x88x923 to 1xc3x971019 cmxe2x88x923 is that effects of improving crystallinity become inferior in case the amount of addition of the impurity is less than 1xc3x971016 cmxe2x88x923 and that insulating characteristics are spoiled in case the value exceeds 1xc3x971019 cmxe2x88x923.
According to the invention as recited in embodiment 29, a temperature of the substrate during the main body film forming process of the invention as recited in embodiment 26 is not more than 400xc2x0 C.
By restricting the temperature of the substrate during the semiconductor film forming process to be not more than 400xc2x0 C., it is possible to prevent dispersion of the impurity including in the substrate and the like into the semiconductor film.
For achieving one of the above objects, the invention as recited in embodiment 30 of the present invention is related to a crystalline semiconductor thin film manufactured through plasma resolution of raw material gas including at least an interfacial film formed on a surface of a substrate and a main body film formed on the interfacial surface, wherein a defect density of the main body film is lower than that of the interfacial film.
According to the invention as recited in embodiment 31, a hydrogen density of the interfacial film of the invention as recited in embodiment 30 is lower than that of the main body film.
For achieving one of the above objects, the invention as recited in embodiment 32 of the present invention is related to a crystalline semiconductor thin film of columnar structure manufactured through plasma resolution of raw material gas, wherein a surface of the semiconductor thin film is smooth.
By the provision of the smooth surface of the semiconductor thin film, it is possible to prevent degradations in characteristics of an entire device in case the thin film is applied to such a device. Note that the term xe2x80x9csmooth surfacexe2x80x9d in the present description denotes a case in which concaves and convexes of the surface are not more than 10 nm.
For achieving one of the above objects, the invention as recited in embodiment 33 of the present invention is related to a thin film transistor using a crystalline semiconductor thin film that has been manufactured through plasma resolution of raw material gas as an active layer, comprising at least an interfacial film formed on a surface of a substrate and a main body film formed on the interfacial film, wherein a defect density of the main body film is lower than that of the interfacial film.
By the above provision, the semiconductor film serving as the active layer exhibits high crystallinity from a proximity of the interface of the substrate, an ON current of the transistor will become high owing to the low defect density of the semiconductor film, and thinning of the film may be easily performed.
According to the invention as recited in embodiment 34, a hydrogen density of the interfacial film of the invention as recited in embodiment 33 is lower than a hydrogen density of the main body film.
For achieving one of the above objects, the invention as recited in embodiment 35 of the present invention is related to a liquid crystal display device in which pixels are driven through a thin film transistor employing a crystalline semiconductor thin film that has been manufactured through plasma resolution of raw gas material as an active layer, the crystalline semiconductor thin film including at least an interfacial film and a main body film, wherein a defect density of the main body film is lower than a defect density of the interfacial film.
In the above arrangement, the semiconductor thin film serving as the active layer of the thin film transistor for driving pixels exhibit high crystallinity from a proximity of the interface of the substrate, and driving performances of pixels as affected by the thin film transistor are improved owing to the low defect density of the film. Thus, it is possible to achieve high image quality of the liquid crystal display device.
For achieving one of the above objects, the invention as recited in embodiment 36 of the present invention is related to a thin film transistor employing a crystalline semiconductor thin film of columnar arrangement that has been manufactured through plasma resolution of raw gas material as an active layer, wherein an upper surface of the crystalline semiconductor thin film of columnar structure is smooth.
By the provision of a smooth upper surface of the crystalline semiconductor thin film as in the above arrangement, it is possible to increase the ON current and to improve reliability of the thin film transistor.
For achieving one of the above objects, the invention as recited in embodiment 37 of the present invention is related to a method for manufacturing a thin film transistor employing a crystalline semiconductor thin film of columnar arrangement that has been manufactured through plasma resolution of raw gas material as an active layer, wherein an upper surface of the semiconductor thin film is smoothed by controlling ion collision energy at the time of depositing the semiconductor thin film through plasma CVD method.
For achieving one of the above objects, the invention as recited in embodiment 38 of the present invention is related to a liquid crystal display device in which pixels are driven by a thin film transistor employing a crystalline semiconductor thin film of columnar arrangement that has been manufactured through plasma resolution of raw gas material as an active layer, wherein an upper surface of the crystalline semiconductor thin film of columnar structure is smooth.
In case at least the upper surface of the crystalline semiconductor thin film is smooth as in the above arrangement, it is possible to achieve improvements in driving performances of the pixels and reliability of the thin film transistor so that it is possible to provide a liquid crystal display device of high image quality.
For achieving one of the above objects, the invention as recited in embodiment 39 of the present invention is related to a thin film transistor employing a crystalline semiconductor thin film that has been manufactured through CVD method as an active layer, wherein at least an upper surface of a channel portion comprised of the crystalline semiconductor film is covered by an oxidation-preventing film.
By covering at least the upper surface of the channel portion by the oxidation-preventing film, it is possible to decrease the amount of oxygen impurities being intermixed into the semiconductor film. Thus, it is possible to increase the ON current of the thin film transistor and to decrease the OFF current.
According to the invention as recited in embodiment 41, the oxidation-preventing film of the invention as recited in embodiment 39 is a silicon nitride film.
For achieving one of the above objects, the invention as recited in embodiment 42 of the present invention is related to a method for manufacturing a thin film transistor employing a crystalline semiconductor thin film that has been manufactured through CVD method as an active layer, wherein an oxidation-preventing film is formed successively after depositing the crystalline semiconductor thin film without exposure to atmosphere.
In the above method, it is possible to prevent intermixing of oxygen impurities into the semiconductor thin film.
For achieving one of the above objects, the invention as recited in embodiment 43 of the present invention is related to a liquid crystal display device in which pixels are driven by a thin film transistor employing a crystalline semiconductor thin film that has been manufactured through CVD method as an active layer, wherein at least an upper surface of at least the crystalline semiconductor thin film comprising the thin film transistor for driving the pixels is covered by an oxidation-preventing film.
In case at least the upper surface of the semiconductor thin film is covered by the oxidation-preventing film as in the above arrangement, it is possible to reduce intermixing of oxygen impurity into the semiconductor thin film so that it is possible to achieve improvements in driving performances of pixels and retaining characteristics of the thin film transistor and thus to provide high image qualities of the liquid crystal display device.
For achieving one of the above objects, the invention as recited in embodiment 44 of the present invention is related to a solar cell employing a crystalline semiconductor thin film of columnar arrangement as a photoelectric converting layer, wherein an upper surface of the crystalline semiconductor thin film comprising a main body layer of intrinsic semiconductors is covered by an oxidation-preventing film that is not of columnar structure.
In case the upper surface of the semiconductor thin film of the intrinsic semiconductor layer that serves as the photoelectric converting layer is covered by the oxidation-preventing film that is not of columnar structure, it is possible to reduce intermixing of oxygen impurity to the intrinsic semiconductor layer so that a converting efficiency of the solar cell may be improved.
For achieving one of the above objects, the invention as recited in embodiment 45 of the present invention is related to a method for manufacturing a solar cell comprising a crystalline semiconductor thin film of columnar arrangement, wherein an oxidation-preventing film that is not of columnar structure is successively formed after depositing the crystalline semiconductor the film without exposure to atmosphere.
According to the invention as recited in embodiment 46, the raw material gas containing compositional elements of the semiconductor thin film of the invention as recited in embodiment 4 is diluted by hydrogen gas or inert gas.
In case the raw material containing compositional elements of the semiconductor thin film is diluted by hydrogen gas or inert gas as in the above arrangement, it is possible to easily control the film forming speed or crystallinity.
According to the invention as recited in embodiment 47, the temperature of the substrate at the time of forming the semiconductor thin film in the invention as recited in embodiment 4 is set to be 100 to 600xc2x0 C.
By setting the temperature of the substrate at the time of forming the semiconductor thin film to be not more than 600xc2x0 C., it is possible to use substrates of low costs such as those made of glass.
According to the invention as recited in embodiment 48, the semiconductor thin film of the invention as recited in embodiment 4 is either micro-crystalline silicon or polycrystalline silicon.
According to the invention as recited in embodiment 49, the semiconductor thin film of the invention as recited in embodiment 1 is either micro-crystalline silicon or polycrystalline silicon.
According to the invention as recited in embodiment 50, the semiconductor thin film of the invention as recited in embodiment 1 is either micro-crystalline silicon or polycrystalline silicon.
In case the semiconductor thin film is comprised of micro-crystalline silicon or polycrystalline silicon, the field-effect mobility of the TFT will be higher than compared to that of a-Si:H films. Note that the terms xe2x80x9cmicro-crystalline siliconxe2x80x9d or xe2x80x9cpolycrystalline siliconxe2x80x9d indicate silicon other than amorphous silicon and single-crystalline silicon.
According to the invention as recited in embodiment 51, the substrate of the invention as recited in embodiment 4 is a light-transmitting type substrate.
According to the invention as recited in embodiment 52, the substrate of the invention as recited in embodiment 1 is a light-transmitting type substrate.
According to the invention as recited in embodiment 53, the substrate of the invention as recited in embodiment 1 is a light-transmitting type substrate.
In case a light-transmitting type substrate is employed as the substrate, it will be favorable in view of manufacturing light-transmitting products such as transmitting-type liquid crystal displays or the like.
According to the invention as recited in embodiment 54, a buffer layer is provided on the surface of the substrate of the invention as recited in embodiment 4.
According to the invention as recited in embodiment 55, a buffer layer is provided on the surface of the substrate of the invention as recited in embodiment 1.
According to the invention as recited in embodiment 56, a buffer layer is provided on the surface of the substrate of the invention as recited in embodiment 1.
By the provision of the buffer layer on the surface of the substrate, it is possible to prevent dispersion of the impurity included in the substrate into the semiconductor thin film, and further to improve adhesiveness of the film.
According to the invention as recited in embodiment 57, a thickness of the semiconductor thin film of the invention as recited in embodiment 1 is not less than 20 nm and not more than 500 nm.
By this arrangement, it is possible to the employ the thin film as a TFT by maintaining its original film thickness and without additionally performing etching or film forming.
According to the invention as recited in embodiment 58, laser annealing of the semiconductor thin film of the invention as recited in embodiment 4 is performed.
According to the invention as recited in embodiment 59, laser annealing of the semiconductor thin film of the invention as recited in embodiment 37 is performed.
With this arrangement, it is possible to obtain a semiconductor thin film and a thin film transistor exhibiting even higher performance.